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Biomedical subjects

R D Fields

Publications and source records attributed to R D Fields.

At least 19 recordsLinked to original sources
PubMed1997

Chronically injured posterior cruciate ligament: magnetic resonance imaging.

Magnetic resonance imaging has been said to be highly reliable for diagnosis of acute posterior cruciate ligament insufficiency. In the present study, 13 patients whose posterior cruciate ligament insufficiency had been documented by magnetic resonance imaging within 10 weeks of the acute injury were recalled for a followup examination and magnetic resonance imaging. The followup interval ranged from 5 months to 4 years. In only 23% of the cases did the posterior cruciate ligament still appear discontinuous on followup magnetic resonance imaging. In the remaining 77%, the posterior cruciate ligament was continuous from tibia to femur, although it appeared abnormally arcuate or hyperbuckled. Conventional interpretation of these magnetic resonance images would suggest that the posterior cruciate ligament had healed. Nevertheless, by clinical examination results, these same patients all were judged to have posterior cruciate ligament insufficiency. Thus, it was concluded that although magnetic resonance imaging may be reliable for evaluation of acute posterior cruciate ligament injury, magnetic resonance imaging findings should not be used to infer functional status in chronic cases.

Chronic Disease↗
PubMed1996

Neural cell adhesion molecules in activity-dependent development and synaptic plasticity.

Cell adhesion molecules (CAMs) have a vital role in forming connections between neurons during embryonic development. Increasing evidence suggests that CAMs also participate in activity-dependent plasticity during development and synaptic plasticity in adults. Neural impulses of appropriate patterns can regulate expression of specific CAMs in mouse neurons from dorsal-root ganglia, alter cell-cell adhesion and produce structural reorganization of axon terminals in culture. Synaptic plasticity in Aplysia, learning in chick and long-term potentiation in rat hippocampus are accompanied by changes in CAM expression. Long-term potentiation can be blocked by disrupting CAM function in rat hippocampus, and learning deficits result from antibody blockade of CAMs in chicks and in transgenic mice lacking specific CAMs. Cell adhesion molecules might produce these effects by controlling several cellular processes, including cell adhesion, cytoskeletal structure and intracellular signaling.

Animals↗
PubMed1996

Modulation of calcium currents by electrical activity.

Electrical activation of mouse dorsal root ganglion (DRG) neurons in cultures for 1-2 days produced a downregulation of voltage sensitive calcium currents, which persisted for > or = 24 h after stimulation was terminated. This regulation varied with different patterns of activation. Both the magnitude and time course of regulation of the low-threshold voltage-activated (LVA) and high-threshold voltage-activated (HVA) currents were differentially sensitive to neural impulse activity. Tonic stimulation at 0.5 Hz did not affect the HVA currents, but 2.5 Hz did produce a significant decrease. Phasic stimulation (10 Hz for 0.5 s every 2 s) with an average frequency of 2.5 Hz produced significantly more downregulation of HVA currents than did the tonic 2.5-Hz stimulation. The efficacy of phasic stimulation varied inversely with the interval between bursts. Thus phasic stimulation of 10 Hz for 0.5 s but delivered every 4 s produced no effects on HVA currents. Stimulation optimal for downregulation of Ca2+ currents also produced a decreased binding by the DRG neurons of an L-type Ca2+ channel antagonist. This suggests a downregulation by electrical activity of the number of Ca2+ channels, rather than an alteration in a constant number of channels. Depression of LVA currents was produced by all stimulus patterns tested, including 0.5-Hz tonic stimulation. Chronic stimulation with a stimulation pattern that downregulated Ca2+ currents also produced a slowing of the increase in intracellular Ca2+ (as measured by Fura-2/AM) that is produced acutely by repetitive stimulation. This is consonant with earlier studies of intracellular Ca2+ concentration kinetics in growth cones.

Animals↗
PubMed1995

Regulated expression of the neural cell adhesion molecule L1 by specific patterns of neural impulses.

Development of the mammalian nervous system is regulated by neural impulse activity, but the molecular mechanisms are not well understood. If cell recognition molecules [for example, L1 and the neural cell adhesion molecule (NCAM)] were influenced by specific patterns of impulse activity, cell-cell interactions controlling nervous system structure could be regulated by nervous system function at critical stages of development. Low-frequency electrical pulses delivered to mouse sensory neurons in culture (0.1 hertz for 5 days) down-regulated expression of L1 messenger RNA and protein (but not NCAM). Fasciculation of neurites, adhesion of neuroblastoma cells, and the number of Schwann cells on neurites was reduced after 0.1-hertz stimulation, but higher frequencies or stimulation after synaptogenesis were without effect.

Animals↗
PubMed1995

Neural activity, neuron-glia relationships, and synapse development.

There is considerable evidence for elimination of synapses and loss of neurons during development of the nervous system. Electrical activity in developing neural circuits induces functional and structural refinement of many synaptic connections, but it is unclear whether the fundamental mechanism is one of strengthening appropriate synapses, combined with the regression of synapses that fail to become adequately stabilized, versus a mechanism of elimination that specifically acts on inappropriate connections. A model of selective synapse elimination, based on the activity-dependent release of proteases and glial-derived protease inhibitors, is presented and supported by evidence from an in vitro preparation of the mouse neuromuscular junction.

Animals↗
PubMed1994

Proteolytic action of thrombin is required for electrical activity-dependent synapse reduction.

Molecular mechanisms of activity-dependent synapse reduction were studied in an in vitro mammalian neuromuscular preparation. Synapse reduction in this model is activity-dependent and is substantially reduced by the broad-spectrum protease inhibitor, leupeptin, suggesting the role of activity-dependent proteolytic action in the process. Our present experiments show that a potent and specific thrombin inhibitor, hirudin, at nanomolar concentration completely blocked the activity-dependent synapse reduction. Furthermore, a naturally occurring serine protease inhibitor, protease nexin I (PNI), which closely colocalizes with acetylcholine receptors at the neuromuscular junction, inhibited the synapse reduction at the same low concentration. In contrast, neither cystatin, a cysteine protease inhibitor, nor aprotinin, a serine protease inhibitor that does not inhibit thrombin, blocked the synapse reduction. Similarly, neither of the inhibitors of the calcium-activated proteases calpain I and II prevented the reduction of synapses. These results strongly suggest that serine proteolytic action by thrombin or thrombin-like molecules is required for synapse reduction in our in vitro model of the mammalian neuromuscular junction.

Amyloid beta-Protein Precursor↗
PubMed1994

Resonant activation of calcium signal transduction in neurons.

The relevant parameters of calcium fluxes mediating activation of immediate-early genes and the collapse of growth cones in mouse DRG neurons in response to action potentials delivered in different temporal patterns were measured in a multicompartment cell culture preparation using digital fluorescence videomicroscopy. Growth cone collapse was produced by trains of action potentials causing a large rise in [Ca2+]i, but after chronic exposure to patterned stimulation growth cones regenerated and became insensitive to the stimulus-induced increase in [Ca2+]i. Calcium reached similar peak concentrations, but the [Ca2+]i increased more slowly than in naive growth cones (time constant of 6.0 s versus 1.4 s in naive growth cones). Semiquantitative PCR measurements of gene expression showed that pulsed stimulation delivered at 1-min intervals for 30 min induced expression of c-fos, but the same total number of action potentials delivered at 2-min intervals failed to induce c-fos expression, even though this stimulus induces a larger peak [Ca2+]i than the effective stimulus pattern. The experiments suggest that the kinetics of calcium fluxes produced by different patterns of stimulation, and changes in the kinetics of calcium flux in neurons under different states of activation, are critical in determining the effects of action potentials on growth cone motility or expression of IE genes during development of neuronal circuits. We propose that differences in kinetics of individual reactions in the stimulus-response pathway may lead to resonance of activation in the neuron, such that certain processes will be selectively activated by particular temporal patterns of stimulation.

Action Potentials↗
PubMed1994

Proteolytic activity, synapse elimination, and the Hebb synapse.

The Hebb synapse has been postulated to serve as a mechanism subserving both regulation of synaptic strength in the adult nervous system (long-term potentiation and depression) and developmental activity-dependent plasticity. According to this model, pre- and postsynaptic temporal concordance of activity results in strengthening of connections, while discordant activity results in synapse weakening. Evidence is presented that proteases and protease inhibitors may be involved in modification of synaptic strength. This leads to a modification of the Hebb assumptions, namely that postsynaptic activity results in protease elaboration with a consequent general reduction of synaptic connections to the active postsynaptic element. Further, presynaptic activity, if strong enough, induces local release of a protease inhibitor, such as protease nexin I, which neutralizes proteolytic activity and produces a relative preservation of the active input. This formulation produces many of the effects of the classical Hebbian construction, but the protease/inhibitor model suggests additional specific mechanistic features for activity-dependent plasticity.

Amyloid beta-Protein Precursor↗
PubMed1993

Effects of electrical stimulation on GAP-43 expression in mouse sensory neurons.

Effects of electrical activity on GAP-43 expression were tested in mouse dorsal root ganglion (DRG) neurons subjected to electrical stimulation in culture. Patterned electrical stimulation was provided through extracellular electrodes placed in multicompartment cell culture chambers. Stimulation was delivered at 10 Hz, in 0.5 s bursts every 2 s for up to 3 days. Expression of GAP-43 was assessed by immunocytochemistry, two ELISA methods, and Northern blot analysis within three experimental protocols: (1) prior to synaptogenesis, (2) after synaptogenesis with spinal cord neurons, and (3) within the context of activity-dependent synaptic competition, in which synapses from active and inactive DRG neurons converge on the same postsynaptic neurons. None of the stimulation treatments produced a measurable change in GAP-43 or RNA message for the protein, although this electrical stimulus induces persistent changes in synaptic strength, and alters neurite outgrowth in these cultures. The decline in GAP-43 levels between 1 and 3 weeks in culture, which has been reported in other studies, was readily detectable by our measurements. We conclude that regulation of GAP-43 expression is not required for activity-dependent regulation of growth cone motility, synaptogenesis and synapse elimination, or changes in synaptic strength. Instead, post-translational modification, such as phosphorylation, may be the primary means of regulating any GAP-43 functions associated with these activity-dependent processes.

Animals↗
PubMed1993

Specific regulation of immediate early genes by patterned neuronal activity.

Electrical activity shapes development of the nervous system, presumably in part by regulating gene expression. A set of regulatory genes, immediate early genes (IEGs), which are responsive to a number of extrinsic cellular stimuli have been proposed to play a role in coupling such activity to gene expression. Using a semiquantitative polymerase chain reaction assay, we show that in dissociated mouse dorsal root ganglion neurons the expression of two IEGs, c-fos and nur/77, is differentially sensitive to patterns of electrical stimulation. Differences in c-fos activation did not correlate with the peak intracellular calcium [Ca++]i produced by the different stimulation patterns or with residual [Ca++]i following stimulation. However, the net increase in [Ca++]i (calcium time integral) was greater for the pulsed stimulus that activated c-fos (6 impulses/min), compared to the ineffective stimulus (12 impulses/2 min). This system of genes seems suited to mediating the coupling between electrical activity and other functional genes.

Action Potentials↗
PubMed1993

Accommodation of mouse DRG growth cones to electrically induced collapse: kinetic analysis of calcium transients and set-point theory.

Electrical stimulation causes growth cones of mouse dorsal root ganglion neurons to collapse. During chronic stimulation, however, growth cones resume motility. In addition, these growth cones are now resistant to the collapsing effects of subsequent stimulation, a process we term accommodation. We compared the kinetics of electrically induced Ca2+ transients in naive and accommodated growth cones in order to determine whether the accommodation process results from a change in the Ca2+ transient, or a change in the Ca2+ sensitivity of the growth cones. Three kinetics were determined: (1) the initial increase to peak Ca2+ levels produced by 10 Hz stimulation; (2) recovery from peak Ca2+ levels during stimulus trains lasting 15 min; and (3) clearing of Ca2+ from growth cones after terminating the stimulus. These kinetics were analyzed using single exponential fits to changes in fura-2 fluorescence ratios. The electrically evoked increase in Ca2+ was significantly slower in accommodated growth cones (tau = 6.0 s) compared to naive growth cones (tau = 1.4 s). Despite the slower increase of [Ca2+]i in accommodated growth cones, peak [Ca2+]i was similar to that reached in naive growth cones, and the steady-state Ca2+ level was significantly elevated after chronic stimulation. Thus, accommodated growth cones maintained outgrowth at [Ca2+]i that caused collapse initially. Time course experiments show that accommodation is a slow process (t 1/2 = about 3 h). Accommodation did not induce measurable changes in the rates of Ca2+ homeostasis during or after stimulus trains. The kinetics of Ca2+ recovery during (tau = 90 s) and after 15 min of stimulation (tau = 8.5 s) was not significantly different in accommodated versus naive growth cones. Rates of 45Ca2+ efflux were also similar in both types of growth cones. These results suggest two regulatory processes contributing to growth cone motility during chronic stimulation: (1) recovery of [Ca2+]i to levels permissive to neurite outgrowth, and (2) an increase in the range of optimal [Ca2+]i for growth cone motility. These adaptive responses of mammalian growth cones to chronic stimulation could be involved in the modulation of CNS development by electrical activity of neurons.

Adaptation, Physiological↗
PubMed1993

Synapse elimination from the mouse neuromuscular junction in vitro: a non-Hebbian activity-dependent process.

The effect of action potentials on elimination of mouse neuromuscular junctions (NMJ) was studied in a three-compartment cell culture preparation. Axons from superior cervical ganglion or ventral spinal cord neurons in two lateral compartments formed multiple neuromuscular junctions with muscle cells in a central compartment. The loss of synapses over a 2-7-day period was determined by serial electrophysiological recording and a functional assay. Electrical stimulation of axons from one side compartment during this period, using 30-Hz bursts of 2-s duration, repeated at 10-s intervals, caused a significant increase in synapse elimination compared to unstimulated cultures (p < 0.001). The extent of homosynaptic and heterosynaptic elimination was comparable, i.e., of the 226 functional synapses of each type studied, 111 (49%) of the synapses that had been stimulated were eliminated, and 87 (39%) of unstimulated synapses on the same muscle cells were eliminated. Also, simultaneous bilateral stimulation caused significantly greater elimination of synapses than unilateral stimulation (p < 0.005). These observations are contrary to the Hebbian hypothesis of synaptic plasticity. A spatial effect of stimulus-induced synapse elimination was also evident following simultaneous bilateral stimulation. Prior to stimulation, most muscle cells were innervated by axons from both side compartments, but after bilateral stimulation, muscle cells were predominantly unilaterally innervated by axons from the closer compartment. These experiments suggest that synapse elimination at the NMJ is an activity-dependent process, but it does not follow Hebbian or anti-Hebbian rules of synaptic plasticity. Rather, elimination is a consequence of postsynaptic activation and a function of location of the muscle cell relative to the neuron. An interaction between spatial and activity-dependent effects on synapse elimination could help produce optimal refinement of synaptic connections during postnatal development.

Action Potentials↗
PubMed1993

Ampullary sense organs, peripheral, central and behavioral electroreception in chimeras (Hydrolagus, Holocephali, Chondrichthyes).

Ampullary sense organs are distributed in groups over the head of Hydrolagus colliei with their pores in clusters and innervated by the buccal, hyomandibular and superficial ophthalmic branches of the anterior lateral line nerve. The ampullae contain ciliated sense cells in an alveolate-shaped epithelium, which communicates to the surface through a jelly-filled tube. The sense cells synapse at their bases with the afferent nerve fibers that terminate in the dorsal nucleus of the anterior lateral line lobe of the medulla. The anatomy and ultrastructure support the homology with the ampullae of Lorenzini of elasmobranchs. Single units recorded from the buccal branch of the anterior lateral line nerve are either lateral line or ampullary in character, the former being sensitive only to mechanical stimuli, the latter to both mechanical and to weak electric stimuli. They are also distinguished by the positions of their receptive fields. The electroreceptive units are spontaneously active and are excited by a cathode placed near the opening of their pore and inhibited by an anode. Compound evoked potentials are recorded from beneath the lateral aspect of the tectum in response to weak electric fields in the bath. Each recording locus has a best position and orientation of the electric field. The electric fields are effective if their duration is longer than ca. 2 ms; longer than 10 ms makes no difference until an OFF effect becomes distinct at ca. 50 ms. The reception is tuned to low frequencies but is not sensitive to maintained current (DC). Evoked potentials summating moderate numbers of responses are clear at < 1 microV/cm. Ratfish were conditioned in a ring-shaped tank to reverse the direction of swimming when an electric field was switched ON. The stimulus was a 5 Hz square wave or the onset of a DC of 1-10 microA between a pair of electrodes on the floor of the tank. The fish responded to fields as weak as 0.2 microV/cm. A specialized sense modality for electroreception, similar to that in elasmobranchs and most other groups of nonteleost fishes, except for Myxini and Neopterygii (holosteans), is present in the subclass Holocephali. The notion is supported that this modality and its central as well as peripheral apparatus arose early in the evolution of vertebrates. Only two losses of the whole system need be hypothesized, on this idea, once in the ancestors of the hagfishes and once in the ancestors of the neopterygians, which include the teleosts. Some orders of teleosts then evolved a new system of electroreception independently. The ciliary receptor cells are probably primitive; microvillar sense cells evolved independently.

Animals↗
PubMed1991

Calcium, network activity, and the role of NMDA channels in synaptic plasticity in vitro.

Functionally effective neuronal circuits are constructed through a competitive process that requires patterned neuronal activity elicited by structured input from the environment. To explore the mechanisms of this activity-dependent synaptic restructuring, we have developed an in vitro preparation of mouse spinal cord neurons maintained in a 3-chambered cell-culture system. Sensory afferents that received chronic electrical stimulation for 3-5 d developed stronger synaptic connections than unstimulated afferents converging onto the same postsynaptic spinal cord neuron. Exposure to 100 microM DL-2-amino-5-phosphonovaleric acid (APV), an antagonist of the NMDA channel, during the stimulation period prevented the competitive advantage associated with electric stimulation. However, when APV was applied with a higher concentration of calcium (3 mM), activity-dependent synaptic plasticity was no longer inhibited by the NMDA receptor antagonist. This reversal of APV block of the plasticity was not impaired by reducing transmitter release with 3 mM magnesium (in addition to 3 mM calcium and APV). A suppressant effect of APV on spontaneous activity was observed, which was attributed to loss of the NMDA component of the EPSP. Activity-dependent plasticity was also blocked if spontaneous activity was suppressed with dilute tetrodotoxin (TTX; 5-10 nM), a dosage that reduces excitability of neurons but is insufficient to block sodium-dependent action potentials. These experiments bring into question how NMDA channel activation is involved in the processes of synaptic remodeling during development. The data suggest that postsynaptic activity is required for synaptic remodeling, but this activity need not involve NMDA receptor activation specifically for activity-evoked synaptic plasticity. Instead, the mechanism for plasticity appears to operate through calcium-dependent processes in general.

2-Amino-5-phosphonovalerate↗